Chair back



FIG. 1 is a perspective view of the chair back according to my design, the broken lines of the portable, folding chair frame with a chair seat are for illustrative purposes only and form no part of the claimed design;

FIG. 2 is a bottom view of the chair back of FIG. 1;

FIG. 3 is a top view of the chair back of FIG. 1;

FIG. 4 is front view of the chair back of FIG. 1;

FIG. 5 is a right side view of the chair back of FIG. 1, the left side being a mirror image thereof; and,

FIG. 6 is a back view of the chair back of FIG. 1. 

I claim the ornamental design for a chair back, as shown and described. 